1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer and an MFP that forms an image on a recording sheet based on print data sent from a host and, more particularly, to an image forming apparatus, image forming system, a fixing control method for carrying out the control of reducing the power consumed for heat-fixing an image formed on a recording sheet with toner to a bare minimum, and a computer program product for executing such control.
2. Description of the Related Art
In an image forming apparatus such as a printer and an MFP that forms an image on a recording sheet based on print data sent from a host, such as a personal computer (PC) with a printer driver installed, a so-called laser printer has been widely used as a printer engine. The laser printer scans and exposes with a laser beam, the lighting of which is controlled in response to image data drawn, to form an image in electrostatic photography. In the course of image forming in this electrostatic photography, the process of fixing, in which a toner image formed on a recording sheet is fixed onto the sheet, requires heating at a high temperature. The power consumed by a heater used for this is substantially large.
In recent years, a demand for reducing such power consumption to a minimum to achieve power saving with reference to a typical electricity consumption (TEC) value has been increasing.
Conventional technologies proposed for the purpose of reducing the power consumed in the fixing includes, for example, Japanese Patent Application Laid-open No. 2009-151102 and Japanese Patent Application Laid-open No. H6-19257. Japanese Patent Application Laid-open No. 2009-151102 discloses that, when a fixing temperature in power saving mode is set, a fixable amount of toner applied is changed corresponding to the fixing temperature. Japanese Patent Application Laid-open No. H6-19257 discloses that a low fixing temperature is set in response to the instruction of power saving mode to reduce the power consumption.
Meanwhile, in many of laser printers and MFPs provided with laser printers, a method to use dithering is adopted to draw an image in halftone. Fixing a toner image, which is transferred to a recording sheet using image data generated by dithering in a drawing process, onto the surface of the sheet requires a high temperature.
This is because, when the shape of the toner image to be fixed on the recording sheet is a small independent dot, there is a high possibility of the dot coming off after printing unless being fixed at a sufficiently high temperature. In contrast, when the shape of the toner image is a line sufficiently long or an area sufficiently wide, the toner image is less likely to come off even when a fixing temperature is relatively low and such fixing is proved to be of practical through experiences.
In such a conventional fixing process, however, there is no conception of varying the control of fixing temperature depending on the difference in type of dither used for drawing images (in terms of this, there is no description made in either of Japanese Patent Application Laid-open No. 2009-151102 and Japanese Patent Application Laid-open No. H6-19257). Thus, the fixing temperature is set to a high temperature adapting to the characteristics specific to small independent dots of the dithering, resulting in the toner image in the shapes of a sufficiently long line and a sufficiently wide area being heated at an unnecessarily high temperature.